Increasing evidence indicates that blood-brain barrier dysfunction contributes to cognitive decline in Alzheimer's disease (AD). Two key elements of barrier dysfunction include 1) reduced levels of the blood-brain barrier trans- porter P-glycoprotein (P-gp) that clears A? from the brain, and 2) barrier leakage, both of which have been linked to A? brain accumulation and cognitive impairment. While increasing evidence shows A? involvement in barrier dysfunction, the underlying mechanisms remain to be fully defined. Moreover, therapeutic strategies to restore barrier function are currently not available. Thus, there is an unmet critical need to define the mechanism(s) that lead/s to barrier dysfunction and to develop effective intervention strategies to help restore barrier function in AD. Absent effective strategies, achievement of therapeutic advances in AD will likely remain challenging. The long-term goal of the investigator is to identify molecular pathways involved in regulating blood-brain barrier function that, when disrupted, contribute to brain disease, and that can be exploited for developing new thera- peutic strategies. The overall objective in this application is to establish a novel combination therapy for AD by targeting the underlying causes of barrier dysfunction in a mouse AD model to restore barrier function and slow the rate of cognitive decline. Based on preliminary data the central hypothesis of this project is that restoring blood-brain barrier function will lead to lower brain A? levels and slow the rate of cognitive decline in humanized PXR/APP mice. The rationale for this research is that providing proof-of-concept that restoring barrier function will reduce cognitive decline in experimental animals would establish a framework for future clinical trials in AD patients. This hypothesis will be tested by pursuing three specific aims: 1) Identify the signaling pathway responsible for A?-mediated P-gp reduction at the blood-brain barrier. 2) Determine the underlying mechanism(s) responsible for A?-mediated barrier leakage, and 3) Develop a combination therapy to restore barrier function in a mouse AD model. Under Aim 1, the mechanism of A?-mediated reduction of P-gp protein expression and activity levels will be assessed in isolated healthy human brain capillaries. Under Aim 2, key signaling steps that trigger barrier leakage will be determined in brain capillaries from healthy individuals and from AD patients. Under Aim 3, the therapeutic benefit of a combination therapy involving PXR activation and scavenging reactive oxygen species to restore barrier function, lower A? brain burden, and slow the rate of cognitive decline will be assessed in a humanized PXR/APP mouse model. The proposed research is innovative, because it represents a substan- tive departure from the status quo by shifting the focus to a disease-modifying combination therapy aimed at two new molecular targets to restore barrier function and improve clinical symptoms. The proposed research is significant because it holds the promise of a novel therapeutic approach to restore barrier function that has high translational potential for clinical use to advance treatment of AD patients. Ultimately such knowledge likely will be applicable to other conditions with underlying barrier dysfunction.